The Tor Project itself does not offer a traditional .exe installer specifically branded as "Portable" in the same way that PortableApps.com does. However, the standard Windows version offered by the Tor Project is inherently capable of being run portably.
Imagine your data is the core of an onion. As your data packet travels through the Tor network, it is encrypted in multiple layers (like the skin of an onion). At each relay (node) in the network, one layer of encryption is peeled away, revealing only the next destination. The final relay decrypts the innermost layer of the packet and sends it to the destination without revealing the source IP address.
installs to your Programs folder and leaves configuration files on the system drive. Portable Tor Browser keeps everything self-contained.